Charge air coolers are used in conjunction with turbocharged internal combustion engine systems. In such systems, residual energy from the combustion exhaust is recaptured through an exhaust expansion turbine, and the recaptured energy is used to compress or “boost” the pressure of the incoming air (referred to as the “charge air”) being supplied to the engine. This raises the operating pressure of the engine, thereby increasing the thermal efficiency and providing greater fuel economy.
The compression of the charge air using the exhaust gases typically leads to a substantial increase in temperature of the air. Such a temperature increase can be undesirable for at least two reasons. First, the density of the air is inversely related to its temperature, so that the amount of air mass entering the combustion cylinders in each combustion cycle is lower when the air temperature is elevated, leading to reduced engine output. Second, the production of undesirable and/or harmful emissions, such as oxides of nitrogen, increases as the combustion temperature increases. The emissions levels for internal combustion engines is heavily regulated, often making it necessary to control the temperature of the air entering the combustion chambers to a temperature that is relatively close to the ambient air temperature. As a result, cooling of the charge air using charge air coolers has become commonplace for turbocharged engines.
Cooling of the charge air is typically accomplished using either direct ambient air to charge air heat exchangers, or indirect liquid cooled charge air heat exchangers. Usually, and especially when the combustion engine is associated with a vehicle, any waste heat generated must eventually be rejected to the ambient air. In a charge air cooler that is directly cooled by ambient air, the charge air is more easily cooled to a low temperature (i.e. a temperature approaching the ambient air temperature). Packaging such a charge air cooler into a compact system can pose challenges, however, as the flow conduits necessary for routing the charge air are often large in order to avoid undesirable pressure loss, and the directly cooled charge air cooler must be located so that a flow of ambient cooling air can be directed through it.
To that end, it has become commonplace in some compact systems to cool the charge air by rejecting heat to a liquid coolant loop. Such a cooling system can be referred to as indirect charge air cooling, as the heat must be transferred first to a liquid cooling loop and subsequently from the liquid cooling loop to the ambient air. Certain advantages can be provided with such an arrangement. Liquid coolant is typically already available, as the combustion engine itself is typically liquid-cooled. Furthermore, liquid lines are much more compact than the charge air lines and can be easily routed, and much more flexibility with regard to location of the charge air cooler is provided. In some cases, a liquid cooled charge air cooler can be placed at or near the air intake manifold of the engine, greatly simplifying the charge air routing over a directly air-cooled system.
However, the requirement that the heat be transferred twice using such an indirect system (first from the charge air to the liquid coolant, then from the liquid coolant to the ambient air) makes it more difficult to achieve the requisite low charge air temperature at the inlet of the combustion cylinders.